1. Field of the Invention
The present invention relates to a feedback controller which can be utilized in many industrial fields, and can quickly match a controlled variable with a target value.
2. Related Background Art
As a representative system of conventional feedback techniques, a feedback controller which uses a PID compensator shown in FIG. 2 is known. Note that the PID compensator is generally a compensator which combines proportional, integral, and derivative operations. A deviation between a target value r and a controlled variable y obtained by a comparator 3 is an input to a PID compensator 21, and the result of the compensation calculation is output as a manipulated variable u to be supplied to a control object.
PID control (normally, PI since most objects have no gain in a high-frequency range) is practically good control system design means since it is simple, has a high-gain characteristic in a low-frequency range, and so on, and the PID control is used in many fields in practice. However, since the PID control adopts a fixed operation system, control performance obtained by this control is limited, and a required characteristic design cannot be obtained for an object having a certain characteristic.
The PID compensator has a linear gain characteristic having one deflection point as PI and two deflection points as PID when the frequency characteristic is considered by approximation, as shown in FIG. 3. In general, in order to improve the control characteristic, shaping of the frequency characteristic near a phase crossover frequency of an open-loop transfer function is important. In most cases, a problem of characteristic adjustment in this frequency range remains unsolved. More specifically, a delicate characteristic design in this frequency range influences all control effects aiming at an increase in gain, an increase in upper limit of the control band, and provision of a proper attenuation characteristic, and the like.
When a further characteristic improvement is required in a PID controller which has no degree of freedom of fine frequency shaping, a filter element such as a phase compensator is added in a conventional system. However, this method largely depends on the experiences of a designer, and trials and errors as multi-dimensional search processing associated with adjustment of a plurality of parameters are inevitable. This is well known as a problem of classical control techniques.
In characteristic shaping in a specific frequency region, there is no clear guideline as to its relation with the time-domain response, and as a result, a plurality of gain-phase compensation elements must be combined and added. Operations associated with such a design are repetitions of off-line design calculations and simulations, and require skills of the designer. Achieving such a design by only simple design calculations and on-line adjustment on actual machines is thus desired.
As described above, in the conventional techniques based on PID control, design jobs offer very poor prospects and require high operation cost except for simple control problems.
As described above, the conventional techniques cannot easily perform fine characteristic adjustment near a phase crossover frequency on the frequency characteristic. In the conventional technique which must adopt a control characteristic design method with a poor prospect, i.e., must combine various compensation elements based on PID compensation, high cost associated with characteristic adjustment is required.
When high control performance is to be obtained, a controller generally requires a complicated arrangement or high-order compensation calculations. This results in an increase in the number of parameters to be set, and hence, it becomes difficult to set and adjust them. The obtained result exhibits poor control performance, and poor maintainability associated with, e.g., re-adjustment upon aging of a control object is also a problem to be solved.